The present invention relates to pacemakers, and more particularly, to pacemakers that pick up body specific signal containing information concerning a demand for hear performance. Known from the prior art are manifold devices that display at least one sensor arranged inside the body of the patient for receiving a body-specific measurement signal containing information concerning demand for heart performance, in order to derive therefrom, among other things, a signal for influencing the stimulation rate.
Devices in existence until now started out from the fact that there exists a direct and time-correct relationship between the change in the body-specific measurement signal containing information concerning demand for heart performance and the stimulation magnitude to be influenced, as for example the amplitude, the rate or the average frequency of occurrence of signals picked up by a sensor, and the heart rate.
However, it has been shown that a direct processing of this type in the time or frequency domain often does not lead to the desired results.
Therefore, the task underlying the invention is to improve the controlling of stimulation events and, more particularly, of the heart rate.
The task, starting out from a device based on the precharacterizing clause of claim 1, is accomplished by the features given in the characterizing part of claim 1.
The invention includes the technical teaching that in the determination of a magnitude influencing the series of the stimulation signal, also to be observed are those relationships that extend beyond a direct association.
Belonging to this, on the one hand, is a transformation of the time periods to be considered, in such a manner that periodic observation also takes place over past time periods, and that control of the heart rate is checked by the repeated appearance of complex frequency sequences or event sequences that display a specific periodicity or rhythm system. The events themselves are identified by comparison of patterns. The periodicity of the frequency sequences or event sequences here clearly deviates from the natural heart rate, and lies in the range from several seconds up to days or months.
In this manner, it is possible to xe2x80x9cdecodexe2x80x9d a body-specific rhythm system and to make it accessible to the heart stimulation, which rhythm system, to be sure, until now has already played an important role in controlling the vasomotor system of the human body, but could not be used for reconstructing natural heart rhythm.
In the case of the signals to be evaluated, we are dealing with circulation magnitudes whose constant fluctuation in rhythmic variations also contains information concerning demand for heart performance. Here, we are dealing in particular with rhythmic variations of the intra-arterial pressure and/or pressure differences picked up at different measurement points in the vascular system, which pressure differences form a measure for vascular resistance between the measurement points.
In the case of the rhythmic pattern superimposed on the blood pressure, we are dealing with wave trains of different arrangement, which wave trains can be ascertained by their periodicityxe2x80x94and, therewith, via the digital-filter or correlation procedures employed for their detection or by appropriately-matched periodic measurement time-segments, which pick up the current amplitude response and/or the associated frequency pattern.
Associated with the invention is the important advantage that, for the first time, signals that until now were not capable of being detected, or that remained unobserved, co-determine the stimulation rate, so that sensors detecting the physical activity of the patient can be dispensed with.
The use of the device in accordance with the invention for maintaining or reconstructing a natural heart rhythm by generating an electrical stimulation signal relates, in particular, to implantable heart pacemakers for treating bradycardia or tachycardia, as well as rhythm-correcting devices, which are also in use as implantable defibrillators.
If, according to the invention, there is connected to the downstream side of a sensor a processing unit that obtains from the measurement signal a body-specific rhythm signal whose periodicity is at least equal to the periodicity of breathing activity, and thus lies above the periodicity of heart activity, whereby the body-specific rhythm signal forms at least indirectly a control signal that influences the point in time, or the time sequence and/or the point in time, of the stimulation signal, then this means that information is taken from a rhythm signal impressed on the circulation system or the nervous system that is of significance for heart activity. It has been found that these signals are also of significance for the heart activity of man, and a stimulation in correlation to such signals forms at least a physiological supplement to signals that are relevant for heart function and picked up some other way from the patient""s body.
One essential application here consists in particular in a stimulation rate adapted to the patient""s xe2x80x9cinternal clockxe2x80x9d, corresponding to a physiological day-night rhythm. In many cases, however, it is also proves to be favorable if circulation rhythms or other biological rhythms of the patient are followed in the stimulation. A stimulation of this kind, corresponding to a biological rhythm, must not be confused with such xe2x80x9cbiological rhythmsxe2x80x9d as are printed out in tables or predicted in purely arbitrary fashion by computers without closer reference to the actual biological object, in the manner of a horoscope.
More rapid body rhythms occurred in a periodicity that begins with the breathing activity and corresponds to an essentially 6- to 20-second rhythm, in particular a 10-second rhythm. However, these rhythms are, mind you, slower than actual heart activity and are preferably in a frequency reduction ratio thereto.
The selection of the body-specific rhythm signal can be appropriately accomplished by means of digital filtering or use of a correlation technique; also, the synchronization of appropriate phase locked loops (PLL) permits a high sensitivity.
In the case of a digital processing, there also exists the possibility of effecting, in advantageous fashion, a detection or a synchronization of periodic or non-periodic processes by comparison of complete signal patterns within the amplitude and/or frequency range. Here, in particular in the processing unit, selected by means of a time window within a time segment of a predetermined duration comprising at least several seconds, is at least one periodically-occurring signal, or a corresponding portion of a signal, having a characteristic amplitude response or frequency pattern, and derived from the sequence of the appearance of this pattern is the body-specific rhythm signal that influences the time sequence or the point in time of the stimulation signal. The width of the time segments to be used in this processing corresponds here, at most, to the period duration of the expected signals or signal portions. A determination of a control signal based on the similarity or accordance of amplitude or frequency patterns appearing in the time segments includes the advantage that a similarity of the signal portions determining the rhythm frequency is recognized more rapidly and more reliably even in the case of less-frequently repeating periods, and the xe2x80x9cbuild upxe2x80x9d of filter circuits or the like need not be waited upon in order to detect the periodic signal portions. This is particularly convenient in the case of rhythms with a long repeat time.
In this, for detection of coincident signal patterns, stored in particular is the periodic signal, or a corresponding signal portion having a characteristic amplitude response or frequency pattern appearing in the time segment in each case, whereby the pattern of the current time segment is continuously compared with at least one similar periodically-occurring signal recorded earlier, or with a corresponding signal portion having a characteristic amplitude response or frequency pattern and, in coincidence with the later-appearing, periodically-occurring signal, or corresponding signal portion having a characteristic amplitude response or frequency pattern, a control signal is emitted when the degree of conformity of the patterns to be compared with one another exceeds a predetermined value, whereby derived from the signal indicating the conformity is the control magnitude that influences the magnitude of the stimulation. In place of the frequency pattern, it is also possible here to compare the time response of one or several spectral portions in the frequency pattern that was (were) picked up within a time segment (wavelet).
The control magnitude, which influences the time sequence or the point in time of the stimulation signal, can here also be derived from the rate, the average frequency, the temporal change of the rate, or of its average frequency, of the signal indicating conformity, so that there results a statistical compression of the derived information that is drawn upon for the stimulation. A corresponding compression of the information can also be achieved if the information from the amplitude response pattern and from the spectral pattern is evaluated together.
For generating frequency spectra, the time-dependent signal is appropriately subjected to a Fast-Fourier-Transformation (FFT).
In addition to influencing the control signal for control of the time sequence of the stimulation signal by periodic influences, it can also be advantageous to vary the control signal and therewith the stimulation signal by means of a positive or negative random signal. The stimulation frequency of the pacemaker will, therefore, not be set so as to be completely fixed, rather will be varied about the desired frequency value. Even in the healthy human body, the heart rate is subjected to more or less random fluctuations that can have various causes. Such fluctuations should therefore also be capable of being reproduced in a pacemaker. If a relatively low random value is superimposed on the control signal, then also hereby a possible positive feedback coupling in the control circuit can be prevented. If a relatively high random value is superimposed on the control signal, it is then possible hereby to xe2x80x9cfeel one""s way towardxe2x80x9d the hemodynamic loading limit, in which case it is then necessary to ascertain the effects of the variations on a measurement magnitude located in the hemodynamic control circuit. By the varying of the stimulation frequency, it is also possible to determine other variables of the hemodynamic control circuit such as time constants and proportionality of the control magnitudes, which, in turn, can be used for optimizing the parameters of the rate-adaptation algorithm of the pacemaker.
In order to counteract a possible tendency to oscillation of circulation functions within the patient""s body by a phase-synchronous excitation because of the generated heart stimulation, it is useful if means are provided that, upon determination of an excessive rise in the amplitude of the current temporal mean value of the body-specific rhythm signal in comparison to a preceding long-term mean value, change the phase position of the change of the stimulation magnitude, in relation to the course of the temporal change of the determined body rhythm magnitudes, by a predetermined or statistically-determined phase angle.
Coming into consideration as body-specific sensor signals that are subjected to the processing in accordance with the invention are, in particular, periodic portions of the intracardial electrogram, the intracardial or transthoracic impedance, blood temperature, vagal/parasympathetic or baroreceptive nerve signals, intra-arterial pressure or the difference in pressure picked up at two different measurement locations as the peripheral vessel resistance of the parts of the circulation system located between the measurement locations, or the electrochemical cellular ion potential.
Other advantageous developments of the invention are characterized in the dependent claims or are represented in more detail in the following with the aid of the figures, together with the description of the preferred implementation of the invention.